Galaxolide is the synthetic substitute for natural Musk.
Musk is found in scent glands at the base of the abdomen of male musk deer of China and Tibet. It is however illegal to kill the deer for the purpose of extracting Musk. Galaxolide allows perfumers to still get the rich woody scent of musk without having to use the natural one.
For use in perfumes you can use between 1%-60%, but a good average is 10%.